Free space duplexed optical communication with transmitter end multiplexing and receiver end amplification

ABSTRACT

A free space wavelength duplexed system includes first and second terminals. The first terminal includes an optical transmitter and an optical receiver. The optical receiver has a telescope, an optical to electrical converter and an optical amplifier coupled between the telescope and the optical to electrical converter. A method includes the steps of receiving a received optical signal through a telescope, diverting the received optical signal in an optical splitter into an optical amplifier, and transmitting a transmit optical signal through the optical splitter to the telescope. Another method includes the steps of receiving plural received optical signals through a telescope, diverting the plural received optical signals in an optical splitter into an optical amplifier, separating the plural amplified optical signals by wavelength, and transmitting plural transmit optical signals at distinct wavelengths through the optical splitter to the telescope.

This application is a continuation application of U.S. patentapplication Ser. No. 09/849,342, filed May 7, 2001 U.S. Pat. No.6,735,356.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a free space optical communicationlink, and in particular, the invention relates to techniques to spreaderror sources over time intervals and provide redundant channels toreduce the effects of fading

2. Description of Related Art

Known optical communication systems rely on optical fibers betweentransmitter and receiver. However, to establish a system network requireobtaining right of ways and installation of fiber, a time consuming andexpensive process.

Free space optical communication systems are fundamentally differentthan fiber optic systems. Distances are more limited. The media is airand subject to atmospheric disturbances such as fog rain and resultingfades.

Koh and Davidson (“Interleaved Concatenated Coding For The TurbulentAtmospheric Direct Detection Optical Communication Channel”, IEEETransactions On Communications. Vol. 37. No. 6, June 1989, pages648-651) discuss how the direct detection atmospheric opticalcommunication channel is characterized by strong fading light intensitycaused by random variations in the index of refraction light variationsas it propagates through the channel.

In addition, the Jet Propulsion Laboratory of the California Instituteof Technology published in November of 1998 a Technical Support Packageon Multiple Beam Transmission For Optical Communication in November 1998as NASA Tech Brief, Vol. 22, No. 11 from a JPL New Technology ReportNPO-20384. This NASA Tech Brief describes how superposition of mutuallyincoherent beams would reduce deleterious effects of atmosphericturbulence.

SUMMARY OF THE INVENTION

It is an object to the present invention to provide a free spacewavelength duplexed optical communication link that is reduces theeffects of fading.

This and other objects are achieved in a free space wavelength duplexedsystem that includes first and second terminals. The first terminalincludes an optical transmitter, and an optical receiver. The opticalreceiver has a telescope, an optical to electrical converter and anoptical amplifier coupled between the telescope and the optical toelectrical converter.

In an alternative embodiment, a method includes the steps of receiving areceived optical signal through a telescope, diverting the receivedoptical signal in an optical splitter into an optical amplifier, andtransmitting a transmit optical signal through the optical splitter tothe telescope.

In another alternative embodiment, a method includes the steps ofreceiving plural received optical signals through a telescope, divertingthe plural received optical signals in an optical splitter into anoptical amplifier, separating the plural amplified optical signals bywavelength, and transmitting plural transmit optical signals at distinctwavelengths through the optical splitter to the telescope.

The receiver includes diversity reception means to optimally combine thereceived signals.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail in the following descriptionof preferred embodiments with reference to the following figureswherein:

FIG. 1 is a schematic of an optical communication system incorporatingthe presenting invention;

FIG. 2 is a block diagram of a first embodiment of the presentinvention;

FIG. 3 is a block diagram of a second embodiment of the presentinvention;

FIG. 4 is a schematic diagram of a telescope according to the presentinvention; and

FIG. 5 is a block diagram of an encoder section of a transmitteraccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, communication system 10 includes a plurality of nodes,depicted as nodes 12, 14, 16 and 18, that may be located on the tops oftall buildings in metropolitan areas and on towers elsewhere. Each nodeis coupled to a network control system that includes central controller20, land lines 22 and one or more radio towers 24. Radio towers 24communicate with the nodes over wireless links 26. The control systemmay advantageously include a typical cellular telephone system,controller 20 (located at a convenient location) and cell phonetransceiver 46 at each node to direct the operation of communicationsystem 10.

The nodes are configured into a network by a plurality of point-to-pointlinks of which link 30 is typical. Each link, as depicted by link 30,includes a bidirectional (e.g., duplex) free space optical channel.However, in any network, there may be one or more links with only aunidirectional channel.

Each node includes at least one outdoor unit 40 (hereinafter ODU), andtypically a plurality of ODUs. For example, eight ODUs 40 are depictedin FIG. 1 on the top of a building at node 12. Each ODU is coupled toswitch circuit 42 through respective cables 44. Each ODU couples freespace optical signals received over link 30 into cable 44, andpropagates optical signals in a fiber in cable 44 as free space opticalsignals over link 30. Switch circuit 42 is controlled by controller 20through cell phone transceiver 46. Typically switch circuit 42 and cellphone transceiver 46 are part of an indoor unit (IDU).

The free space optical channel (hereinafter FSOC) can transmit at asuper high bandwidth that no other wireless technology can offer.However, the FSOC is subject to transmission beam fades due toatmosphere turbulence. Some or all of the transmission beam fades can bereduced by use of delay and diversity techniques which include redundanttransmission beams and wavelengths within the optical or electrical paththrough a free space optical communication system to overcometransmission errors due to atmospheric fade.

In FIG. 2, a free space optical channel (as in link 30 of FIG. 1)includes transmitter 80 and receiver 90. Transmitter 80 includeselectrical multiplexer 81 to drive laser transmitter 82 as input tooptical amplifier 84 to couple an optical signal through fiberconnection 86 to optical telescope 88. Electrical multiplexer 81 acceptsmultiple independent data signals and combines them into a single signalto modulate laser transmitter 82. Laser transmitter 82 is preferably,but not necessarily, a properly driven laser diode. Optical amplifier 84is preferably, but not necessarily, an erbium doped fiber amplifier(EDFA). Optical amplifier 84 receives the optical signal from lasertransmitter 82 and provides an amplified optical signal at the desiredpower level to fiber connector 86. Fiber connector 86 is preferably, butnot necessarily, a single mode optical fiber to deliver the amplifiedoptical signal from optical amplifier 84 (usually part of the indoorunit) to optical telescope 88 (usually part of the outdoor unit).Optical telescope 88 propagates the optical signal through free space(the atmosphere) from node that includes optical telescope 88 to thenode that includes optical telescope 91.

Receiver 90 includes optical telescope 91 coupled to optical splitter92. The optical signal that passes out of the receive port of opticalsplitter 92 is captured by conical fiber collector 93. Preferably,conical fiber collector 93 is surrounded by four quadrant detectors(e.g., photo diodes) that are sensed electrically to adjust the point ofoptical telescope 91, if necessary however, conical fiber collector 93collects the optical signal for transmission in a fiber to opticalfilter 94. Optical filter 94 ensures that only the desired receiveoptical band is passed to optical amplifier 95; thereby eleminating anyback scatter from optical transmitter 99. Optical amplifier 95 ispreferably, but not necessarily, an erbium doped fiber amplifier (EDFA).The output of optical amplifier 95 is coupled through optical attenuator96 to optical detector 97 (e.g., a photo diode). Optical attenuator 96senses the optical power level and receives a command to adjust theamount of attenuation to ensure that optical detector 97 is alwaysoperated at an optimal power operating point. Dectector 97 converts theoptical signal into an electrical signal that is demultiplexed indemultiplexer 98. Demultiplexer 98 is the conjugate of multiplexer 81 oftransmitter 80.

The link from transmitter 80 to receiver 90 is one direction of a duplexchannel. To implement the other direction, wavelength duplexing is used(transmission in one direction is at a wavelength that is different fromthe transmission in the other direction). This feature enables opticalfilter 94 to block back scatter form optical transmitter 99. The reversedirection transmission originates at optical transmitter 99. Opticaltransmitter 99 preferably includes all of the individual elementsdescribed as multiplexer 81, laser transmitter 82, optical amplifier 84and fiber connection 86. The optical output of optical transmitter 99enters the transmitter port of optical splitter 92, and from there ispropagated to telescope 91 for transmission to telescope 88. An opticalsplitter (that corresponds to optical splitter 92) is coupled betweentelescope 88 and fiber 86 so that a receive optical signal may beprocessed in a way that corresponds to the way described with respect toreceiver 90.

In FIG. 3, a free space optical channel (as in link 30 of FIG. 1)includes transmitter 50 and receiver 60. Transmitter 50 includes firstoptical transmitter 52 and second optical transmitter 54. The inputsignal IN is divided to independently and simultaneously excite firstand second optical transmitters 52 and 54. Typically, each opticaltransmitter is a laser diode but may include other high speed modulatedelectro-optical devices such as light emitting diodes (LEDs). Firstoptical transmitter 52 transmits the input signal carried on wavelengthλ1, and second optical transmitter 54 transmits the input signal carriedon wavelength λ2. Transmitter 50 further includes optical combiner 56and optical telescope 58 to transduce the optical signals from theoutputs of first and second optical transmitters 52 and 54 into freespace optical beams directed in the direction of receiver 60.

Receiver 60 includes optical telescope 62 to transduce the free spaceoptical beams received from transmitter 50 into an optical signal(typically contained in an optical fiber) that is supplied throughoptical amplifier 63 (e.g., erbium doped fiber amplifier) to wavelengthdemultiplexer 64. Wavelength demultiplexer 64 separates wavelengthdivision multiplex optical signals into an optical signal carried onwavelength λ1 and an optical signal carried on wavelength λ2. Theoptical sigal carried on wavelength λ1 is detected inoptical-to-electrical converter 66, and the optical signal carried onwavelength λ2 is detected in optical-to-electrical converter 68. Theoptical-to-electrical converters may be, for example, photodiodes,avalanche photodiodes, phototransistors or photogates the detectedoutputs of converters 66 and 68 are combined in diversity combiner 70,and the combined signal is output as signal OUT.

The optical transmitter and optics of FIG. 3 includes a dual channel (orplural channel) arrangement that converts the input signal intoredundant optical signals at different wavelengths before opticallysending two beams (or plural beams) to the receiver. In this way, anoptical transceiver optically modulates a signal onto redundant channelsat different wavelengths. At the receiver end, the free space signal isconverted to an optical signal in a fiber and amplified in opticalamplifier 63. Prior art transmission systems do not transmit multiplebeams at correspondingly distinct wavelengths while optically amplifyingthe received signal.

At the receiver, signal levels of the optical signals at wavelengths λ1and λ2 are monitored and used to optimally combine the received signals.

In FIG. 4, transmit telescope 120 includes two input fibers carryingoptical signals at two different wavelengths (λ1 and λ2). The twooptical signals are combined in coupler 122 and the combined signal isdivided in coupler 124. From each end 126 of the dividing coupler, amulti-wavelength beam is launched and focused by optical lens 128 on adistant receiving telescope 130. Although the transmit telescope focuseswell its beams, there will be some small dispersion of the beam thatresults in an overlap area. Optical lens 132 of receive telescope ispositioned in the overlap area so that lens 132 receives thesuperimposed beams. Lens 132 focuses the overlaped beams into conicaltaper 134 which collects the optical signal as a multimode signal forfurther processing.

The laser transmitter of FIG. 2 or 3 may be replaced with a wavelengthdivision transmitter of FIG. 5. In FIG. 5, the transmitter includesencoding section ISO. Encoding section 150 includes multiplexer 152 tomultiplex together plural diverse input signals and provide a serialbitstream at its output. Then, in serial to parallel converter 154 theserial bitstream is converted into plural parallel signals (a predefinednumber of signals) to be processed. Each parallel signal is process inparallel section 160 that includes forward forward error correctionencoder 162 (an FEC encoder or other redundance error correctionencoder), bit interleaver 164 and a laser transmitter 166 (e.g., a laserdiode or other laser source). For example, an output of FEC encoder 162might be a signal organized in a block made of 8 bytes with each bytehaving 8 bits. Interleaver 164 might take the first bit of each bytebefore taking the second bit of each byte. In this way, errors arespread out over the time it takes to transmit the block in order to“whiten” the effect of an error and make it easier for a FEC code tocorrect for the error. Each interleaved signal is then converted into anoptical signal on a distinct, predefined wavelength and the opticalsignals are combined in optical combiner 156 (e.g., coupler 122 of FIG.4), and the combined signal is amplified in optical amplifier 158 (e.g.,an erbium doped fiber amplifier, EDFA) before being sent to atransmitter telescope (see FIG. 2 or 3). Prior art does not use thisarrangement for an optical transmitter, and thus is unable to toleratedeep fades (>30 dB) that last for tens of milliseconds.

Having described preferred embodiments of a novel free space opticalcommunications link (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. For example,various techniques of sending redundancy information and redistributinginformation over the time slot for a block of data may be combined towhiten and limit the effects of fading. It is therefore to be understoodthat changes may be made in the particular embodiments of the inventiondisclosed which are within the scope and spirit of the invention asdefined by the appended claims.

Having thus described the invention with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1. A terminal in a free-space wavelength duplexed system comprising: atelescope having an equipment end a free-space end that is adapted tocarry free-space, wavelength duplexed, communication; an opticalsplitter having a first port connected said equipment end of saidtelescope; an optical transmitter for generating an optical signalcomposed of signals developed by modulating each of a plurality of lasersources with an input signal, and each of said laser sources having adistinct wavelength, said signal being coupled to a second port of saidoptical splitter; and an optical receiver coupled to a third port ofsaid optical splitter, which receiver includes an optical amplifier andan optical filter followed by an optical-to-electrical converter, wheresaid optical filter passes an optical signal of a wavelength that isdifferent from any of the wavelengths of said laser sources.
 2. Theterminal of claim 1, wherein: the optical transmitter includes: aserial-to-parallel converter responsive to an input signal, forgenerating an M plurality of signals, an M plural lasers sources, eachlaser source modulating a different one of the signals of said Mplurality of signals signal onto an optical signal having a wavelengththat is distinct from a wavelength of each other laser source, to forman M plurality of different-wavelength optical signals; and an elementthat combines said M plurality of different-wavelength optical signalsto form a combined result, and applies the combined result to saidoptical splitter.
 3. The terminal of claim 2 further comprising an errorcorrection coding circuit interposed between each of saidserial-to-parallel converter and each one of said M plural lasersources.
 4. The terminal of claim 2 further comprising a multiplexer formultiplexing N information-bearing signals to form said input signal. 5.The terminal of claim 1 where the optical receiver includes a WDMdemultiplexer preceding said optical-to-electrical converter.
 6. A freespace optical communication network comprising: a plurality ofterminals, each of which includes a telescope having an equipment endand a free-space end that is adapted to carry free-space, wavelengthduplexed, communication, which telescope is positioned to point to atelescope of another one of said terminals; an optical splitter having afirst port connected said equipment end of said telescope; an opticaltransmitter for generating an optical signal composed of signalsdeveloped by modulating each of at least one laser source having a firstwavelength with an associated information-bearing input signal, saidsignal being coupled to a second port of said optical splitter; and anoptical receiver coupled to a third port of said optical splitter havingan optical-to-electrical converter and an optical amplifier coupledbetween said optical splitter and the optical-to-electrical converter.7. The system of claim 6 where said terminals are located on roofs ofbuildings.
 8. The systems of claim 6 where at least some of saidterminals include an electronics module that is adapted to communicatein a wireless manner with a base station.
 9. The system of claim 8 wheresaid base station includes a radio tower.
 10. The system of claim 6where said terminals form a plurality of subsets, and each subset islocated on one building, having their telescopes pointing to telescopeson other buildings.
 11. The system of claim 10 where each subset ofterminals includes a common electronics module that communicates witheach terminal in said subset.
 12. The system of claim 11 where at leastsome of the electronic modules communicate with a base station in awireless manner.
 13. A method executed in a terminal that is adapted tooperate as a transmitter and a receiver, for communicating with lighttraversing free space comprising the steps of: accepting an informationbearing optical signal at an optical amplifier; passing the acceptedsignal from a first port of a coupler to a second port of the coupler;passing a signal outputted by said second port to a telescope, to beoutputted by the telescope into free space; receiving a signal from freespace by said telescope and passing the received signal to said secondport of said coupler to be passes to a third port of the coupler; andpassing a signal outputted by said third port to an optical toelectrical converter.
 14. The method of claim 13 further comprising astep of generating said information-bearing optical signal from anelectrical signal that modulates an optical carrier signal that isgenerated by a laser.
 15. The method of claim 14 where said electricalsignal includes forward error correction information.
 16. The method ofclaim 13 further comprising a step of generating saidinformation-bearing optical signal by modulating a plurality ofelectrical signals by a corresponding plurality of lasers, each of whichgenerates a carrier signal of a given wavelength.
 17. The method ofclaim 16 where said plurality of electrical signals are generated fromone or more electrical signals.
 18. The method of claim 13 furthercomprising a step of generating said information-bearing signal.
 19. Themethod of claim 13 further comprising a step of filtering the receivedsignal prior to said step of passing.
 20. The method of claim 19 wheresaid step of filtering includes a step of wavelength divisiondemultiplexing of the received signal.
 21. The method of claim 13,further comprising the step of generating said input signal bymodulating an electrical signal with a laser.
 22. The method of claim13, further comprising the step of generating said input signal bymodulating a plurality of electrical signals, each carrying a distinctstream of information, with a corresponding one of a plurality oflasers, each having its own distinct wavelength.